Gas phase lasers provide a unique combination of optical characteristics and performance attributes which make them useful optical sources for a variety of industrial and research applications. HeNe lasers, for example, are a class of gas phase laser capable of providing moderately high powers (e.g., ≈1-35 mW) of radiant output that varies over a narrow frequency range (e.g., on the order MHz to GHz) and exhibiting excellent beam quality (e.g., beam profile nearly ideal Gaussian TEM00). Moreover, HeNe lasers are mechanically robust, provide highly reliable and long optical source lifetimes (e.g., 50,000-100,000 hours of use) and are capable of effective frequency stabilization, commonly without the need for integrated frequency references. HeNe lasers also provide radiant output accessing a wide range of wavelengths. Given this combination of beneficial attributes, gas phase laser optical sources, such as HeNe lasers, are widely used in diverse high performance settings, including sensing, spectroscopy, interferometry, holography and biomedical instrumentation.
Gas phase lasers typically comprise an optical resonant cavity having an integrated gas discharge tube containing a gaseous gain medium. A typical HeNe gas laser, for example, comprises a gas discharge tube enclosing a gain medium comprising a mixture of helium and neon gases. Pumping of the gain medium is commonly achieved via electrical discharge (e.g., dc, ac or rf excitation) which serves to ionize a portion of the mixture of gases initiating a complex series of electron-atom and atom-atom interactions resulting in excitation of Ne atoms of the gas mixture. An optical resonant cavity is provided such that the rate of stimulated emission exceeds the rate of spontaneous emission upon relaxation of excited Ne atoms, thereby providing optical amplification and lasing. HeNe lasers capable of providing collimated, single- or multi-longitudinal mode radiant output in the visible (e.g., 543.5 nm, 594 nm, 604 nm, 612 nm and 632.8 nm), near infrared (e.g., 1152 nm and 1523 nm) and infrared (e.g., 3392 nm) regions of the electromagnetic spectrum have been developed via appropriate selection of resonant cavity configurations including selection of the resonant cavity optical path length, reflector arrangement, and/or reflectivities of reflector optics.
Multi-longitudinal mode gas phase lasers having cavity reflectors with ion beam sputtered optical coatings provide a radiant output comprising a plurality of linearly polarized longitudinal modes having angular orientations of their polarization planes that vary as a function of time. In some dual longitudinal mode HeNe lasers, for example, two linearly polarized longitudinal modes having orthogonal linear polarization states are excited in the laser having angular orientations of their respective polarization planes that vary as a function of time. The output of these lasers, therefore, is characterized by a radiant beam comprising two orthogonal linear polarization states oscillating in polarization planes that spontaneously rotate about the propagation axis of the beam. Uncontrollable rotation of the polarization states of the radiant output of these lasers substantially hinders separation of the two longitudinal modes having orthogonal linear polarization states. This property of some multi-longitudinal mode gas phase lasers is undesirable as it prevents use of these optical sources in polarization dependent optical systems, and makes frequency stabilization of these lasers significantly more difficult. Polarization of the radiant output of gas phase lasers is described in detail in “Lasers and Electro-Optics: Fundamentals and Engineering” C. Davis, Cambridge University Press, 1996, and “Fundamentals of Photonics” Saleh and Teich, Wiley, New York, 1991 which are hereby incorporated by reference in its entirety.
Given the potential benefits of gas phase lasers in a variety of technical settings, substantial research has been directed in developing design strategies for addressing problems associated with spontaneous rotation of the polarization states of the radiant output of these optical sources. A number of these strategies involve incorporation of one or more intracavity optical elements having selected optical properties capable of providing some degree of polarization stabilization and control.
U.S. Pat. No. 6,567,456 describes a gas phase laser configuration wherein an intracavity reflector providing polarization dependent reflectivity is incorporated into the resonant cavity. The intracavity reflector is positioned such that electromagnetic radiation from one end reflector is directed to the other end reflector upon reflection from the intracavity reflector. In an embodiment, the intracavity reflector is highly reflective for s-polarized light and partially reflective for p-polarization. This optical configuration provides differential loss of p-polarized electromagnetic radiation, thereby resulting in lasing of only the s-polarization state. The radiant output of this gas phase laser configuration generates one or more longitudinal modes all linearly polarized along the same direction (e.g., s-polarized orientation). Further, deviations in the angular orientation of the polarization planes of the radiant output are minimized using this configuration. A drawback of this approach, however, is that the system does not simultaneously support multiple polarization states.
Optical resonator design strategies for providing polarization control have also been implemented using an internal Brewster window capable of providing a differential loss of a selected polarization state. Similar to the approach in U.S. Pat. No. 6,567,456, incorporation of the Brewster window prevents lasing of a selected polarization state, thereby generating radiant output comprising one or more linearly polarized longitudinal modes having the same angular orientation of their polarization planes, and exhibiting enhanced stability with respect to deviations in the angular orientation of the polarization plane. This approach is also incompatible, however, with simultaneous support of multiple polarization states.
U.S. Pat. No. 5,097,481 provides a design strategy for providing a dual-longitudinal mode gas phase laser for generating radiant output having fixed angular orientations of its polarization planes. This reference describes an alternate gas phase laser configuration wherein a birefringent element is provided in the beam path of the optical resonant cavity. The birefringent element has two preferred oscillatory planes for linearly polarized electromagnetic radiation which are oriented perpendicular to each other. In an exemplary embodiment, the birefringent element is a vapor deposited layer provided on an end reflector of the resonant cavity wherein the deposition direction is inclined by about 50 degrees through 70 degrees relative to normal incidence with respect to the end reflector surface. The birefringent layer is oriented such that laser electromagnetic radiation is incident to the birefringent layer at normal incidence. Addition of the birefringent layer is reported to result in generation of radiant output comprising two longitudinal modes having orthogonal linear polarization states with angular orientations that do not vary significantly with respect to time. A drawback of this approach, however, is that it requires use of specialized birefringent layers which add to the cost and complexity of the cavity reflectors, and may also deleteriously impact other important optical properties (e.g., reflectivity, degree of collimation/focusing, scattering etc.) of the cavity reflectors.
Another approach to providing a dual mode gas phase laser for generating radiant output with substantially fixed angular orientations of its polarization planes involves use of a resonant cavity having at least one end reflector having an electron-beam deposited coating. This resonant cavity configuration gives rise to polarization stabilization, possibly due to birefringence of the electron-beam deposited coating itself likely due to the directional nature of the structural columnar, growth of e-beam deposited films or directional control of coating stress. Electron-beam deposited coatings, however, are typically regarded as inferior to ion beam sputtered reflectors in regard to overall performance for laser cavity applications. For example, the longevity and stability with respect to externally applied mechanical stress and temperature variations of electron-beam deposited coatings is less than that of ion beam sputtered optical coatings. Also the directions of the axes of polarization of radiant output from two longitudinal mode gas phase lasers having electron-beam deposited coatings must be measured after manufacture, thereby requiring an additional preselection step.
It will be appreciated from the foregoing that multi-longitudinal mode gas phase lasers are needed capable of generating radiant output having controlled and stabilized polarization states, particularly multi-longitudinal mode gas phase lasers having resonant cavity reflectors comprising ion beam sputtered optical coatings. Multi-longitudinal mode gas phase lasers are needed that provide a radiant output comprising longitudinal modes having a plurality of preselected and well defined angular orientations of their respective polarization planes. Further, multi-longitudinal mode gas phase lasers are needed that provide polarization stabilized radiative output comprising longitudinal modes with different linear polarization states that have substantially constant angular orientations of their polarization planes as a function of time. Further, multi-longitudinal mode gas phase lasers having resonant cavity reflectors comprising ion beam sputtered optical coatings are needed that are capable of frequency stabilization.